Emi-absorbing air filter

ABSTRACT

Electro-magnetic-energy absorbing materials are used to treat air filters, such as those used in association with electronic equipment thereby suppressing the transmission of electromagnetic interference (EMI) therethrough. Disclosed are processes and materials for applying EMI-absorbing materials to air filters thereby improving EMI-shielding effectiveness in an economically embodiment, an absorptive solution is prepared using an absorptive material and a binding agent. A heavy coating of absorbing solution applied to an air filter substrate, for example by dipping or spraying. Excess absorbing material is subsequently removed and the absorbing material cured, such that the passage of air through the filter remains substantially unimpeded. The resulting absorptive air filter is then optionally treated with a flame retardant to meet a predetermined safety standard.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to the attenuation ofelectromagnetic energy and, more specifically, to porous materialsincorporating electromagnetic-energy-attenuating materials.

2. Description of the Prior Art

As used herein, the term EMI should be considered to refer generally toboth electromagnetic interference and radio frequency interference (RFI)emissions, and the term “electromagnetic” should be considered to refergenerally to electromagnetic and radio frequency.

During normal operation, electronic equipment typically generatesundesirable electromagnetic energy that can interfere with the operationof proximately located electronic equipment due to EMI transmission byradiation and conduction. The electromagnetic energy can exist over awide range of wavelengths and frequencies. To minimize problemsassociated with EMI, sources of undesirable electromagnetic energy canbe shielded and electrically grounded to reduce emissions into thesurrounding environment. Alternatively, or additionally, susceptors ofEMI can be similarly shielded and electrically grounded to protect themfrom EMI within the surrounding environment. Accordingly, shielding isdesigned to prevent both ingress and egress of electromagnetic energyrelative to a barrier, a housing, or other enclosure in which theelectronic equipment is disposed.

In the abstract, an ideal EMI shield would consist of a completelyenclosed housing constructed of an infinitely conductive-materialwithout apertures, seams, gaps, or vents. Practical applications,however, result in an enclosure constructed of a finitely conductingmaterial having some apertures. Apertures may be unintentional, such asthose incident to a method of construction (for example, gaps or seams,for example between adjacent access panels and around doors, or betweencomponent housings and circuit boards), or intentional, such as vents toaccommodate air flow for cooling. Special methods of manufacture may beemployed to improve the shielding effectiveness of unintentionalapertures, for example, by welding or soldering seams, or by milling acavity within a contiguous member of shielding material, therebyeliminating unintentional apertures.

As mentioned, cooling vents are typically required because electronicequipment typically generates thermal energy (that is, heat) that mustusually be removed from the equipment to ensure continued, long-term,and proper operation. Shielding of apertures relating to cooling ventsare necessarily more challenging, because the apertures themselvescannot be eliminated as cooling air must be allowed to pass through tofacilitate heat transfer.

Prior-art solutions are available that provide some level of EMIshielding across a cooling aperture. For example, a cooling aperture maybe covered by an electrically conducting plate having field of smallerapertures (that is, a two-dimensional array) spanning the coolingaperture. Other solutions include an electrically conductive screen,while still other solutions include a two-dimensional array of waveguideapertures (for example, a “honeycomb”). Each of these solutions providespreferential attenuation to lower-frequency EMI having a frequency belowsome “cutoff” frequency generally determined by the largest dimension ofeach individual aperture. Moreover, these solutions are complicated asthey rely on a positive electrical bonding of the plate or screen to theequipment housing that must be maintained over the life of theequipment. Maintaining such an electrical bond can be particularlychallenging in high-vibration and/or corrosive environments.

As mentioned, shielding effectiveness of such conventional methods andmaterials decreases with increasing frequency. Thus, effective shieldingof EMI in many of today's electronic applications is becoming morechallenging, as current trends continue to increase operationalfrequencies. For example, microprocessor clocking rates used withincurrently available consumer electronics, such as personal computers,are operating at thousands of megahertz. Later generation devices areexpected to operate at even greater frequencies.

There exist other methods for providing EMI shielding across coolingapertures. See, for example, U.S. Pat. No. 5,151,222 issued to Ruffoni,the disclosure of which is herein incorporated by reference in itsentirety. Ruffoni discloses the use of an open-cell reticulatedpolyurethane foam impregnated with a conductive ink. The methoddisclosed in Ruffoni applies the conductive ink to the surface of thefoam, resulting in a variation, or gradient, in conductivity from thecoated surface of the foam to its interior. Purportedly, the conductiveink offers improved attenuation performance at higher frequencies.Unfortunately, however, due to the resulting gradient, a heavierapplication of conductive ink is required at the foam surface in orderto provide a desired overall attenuation characteristic. Such anapplication results in a pressure drop from blocked pores due to aheavier application of conductive ink necessary to meet increasingattenuation requirements. As such, the foam shown in Ruffoni is notsuitable for use as an air filter.

SUMMARY OF THE INVENTION

In general, the present invention relates to anelectromagnetic-interference- (EMI-) (EMI-) absorbing air filter, suchas a planar, porous substrate (for example, a polyurethane foam sheet)treated with an electromagnetic-interference- (EMI-) absorbing material.The EMI-absorbing material absorbs a portion of the EMI incident uponthe treated air filter, thereby reducing transmission of EMItherethrough over a range of operational frequencies. The absorbingmaterial may remove a portion of the EMI from the environment throughpower dissipation resulting from loss mechanisms. These loss mechanismsinclude polarization losses in a dielectric material and conductive, orohmic, losses in a conductive material having a finite conductivity.

Accordingly, in a first aspect; the invention relates to an air filterhaving EMI-absorbing characteristics. The filter includes a poroussubstrate (for example, an open-cell reticulated polyurethane foam) andan EMI-absorbing material applied to the porous substrate. In oneembodiment, the EMI-absorbing material includes particles of anelectrical-energy absorber, such as carbon, and a binding agent, such asan epoxy, or elastomer. In another embodiment, the EMI-absorbing airfilter undergoes multiple applications of EMI-absorbing material duringmanufacture.

In another embodiment, the EMI-absorbing air filter also includes a fireretardant. In yet another embodiment, the EMI-absorbing air filterincludes an electrically conductive layer. The conductive layer can beconfigured, for example, as a screen or as a waveguide-below-cutoffarray (that is, a honeycomb).

In another aspect, the invention relates to a process for fabricating anair filter having EMI-absorptive characteristics. The process includesthe steps of providing a porous substrate having a first side and asecond side and applying an EMI-absorptive material to the poroussubstrate in a manner allowing the electrically-absorptive material tobe distributed substantially constantly or evenly throughout the depthof the porous substrate. In one embodiment, the step of applying theEMI-absorbing material further includes the sub-steps of providing anEMI-absorbing solution including an electrical absorber and a bindingagent, immersing the porous substrate into the EMI-absorbing solutioncausing the solution to penetrate the porous substrate, extracting theporous substrate from the solution, removing any excess solution, andcuring the EMI-absorbing material. In another embodiment, theabove-described procedural steps are repeated one or more times to applyadditional EMI-absorbing material to meet a desired attenuationperformance requirement.

In another embodiment, the step of applying the EMI-absorbing materialincludes providing an electrically absorptive solution including anelectrical absorber and a binding agent; spraying the electricallyabsorptive solution onto a first side of the porous substrate; removingexcess electrically absorptive solution from the sprayed, poroussubstrate, thereby leaving the EMI-absorbing material substantiallyevenly distributed through the porous substrate; and curing theelectrically absorptive material.

In another embodiment, the process of applying a layer of EMI-absorbingmaterial further includes the step of forcing air through the poroussubstrate, thereby ensuring that pores remain substantially unblocked.In yet another embodiment, the process further includes the step ofapplying a fire-retardant layer.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is pointed out with particularity in the appended claims.The advantages of the invention may be better understood by referring tothe following description, taken in conjunction with the accompanyingdrawings, in which:

FIG. 1 is a flow chart diagram depicting procedural steps of anembodiment of a process for fabricating an EMI-absorbing air filter;

FIGS. 2A and 2B are schematic diagrams depicting perspective views ofalternative embodiments of a free-standing EMI-absorbing air filter, anda frame-mounted EMI-absorbing air filter, respectively;

FIG. 3 is a schematic diagram depicting a perspective view of analternative embodiment of an EMI-absorbing air filter combined with aconductive layer;

FIG. 4 is a schematic diagram depicting a perspective view of analternative embodiment of an EMI-absorbing air filter combined with awaveguide-below-cutoff filter;

FIGS. 5A and 5B are schematic diagrams depicting perspective views ofexemplary alternative embodiments of an EMI-absorbing air filter formedin non-planar configurations;

FIG. 6 is a schematic diagram depicting a manufacturing process capableof yielding the EMI-absorbing air filter depicted in FIGS. 1 through 5;

FIG. 7 is a schematic diagram depicting an alternative manufacturingprocess also capable of yielding the EMI-absorbing air filter depictedin FIGS. 1 through 5;

FIG. 8 is a chart depicting test measurement results of the attenuationversus frequency for an exemplary sample of an EMI-absorbing air filter;

FIG. 9 is a schematic diagram depicting an exemplary test setup enablingthe measurement of air flow through a material, such as theEMI-absorbing air filter; and

FIG. 10 is a chart depicting test measurement results of the pressuredrop versus air flow measured using the test setup illustrated in FIG. 9for two exemplary samples of EMI-absorbing air filters in accordancewith the invention compared to an untreated air filter.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Materials having electromagnetic-energy-absorbing properties can be usedto suppress the transmission of EMI over a broad range of frequencies.Such EMI-absorbing materials can provide substantial electromagneticshielding effectiveness, for example, up to about 5 dB or more at EMIfrequencies occurring up to about 100,000 megahertz.

According to the present invention, EMI-absorbing materials can beformed in a solution capable of being applied to a suitable poroussubstrate. Generally, the resulting absorptive solution includes anabsorbing material and a binding agent that can be applied to new,custom air filters, or to commercially available non-EMI air filters.

Referring to FIG. 1, procedural steps are illustrated for one embodimentof a process applying an EMI-absorbing material to an air filter. Inbrief overview, a porous substrate is provided (step 100) along with acurable, EMI-absorbing solution (step 110). Next, the EMI-absorbingsolution is applied to the porous substrate (step 120) followed by theremoval of any excess solution (step 130). The EMI-absorbing solutiondeposited in the porous substrate is then cured (step 140). If a greaterEMI-absorbing performance is required, steps 120 through 140 can berepeated one or more times (step 150), thereby applying additionalEMI-absorbing material. In some embodiments, a fire retardant isoptionally applied (step 160, shown in phantom).

In more detail, the porous substrate is generally selected as havingproperties desirable for an air filter, namely, a high dust arrestanceand a low pressure drop (or, conversely, a high air permeability). Onemeasure of the porosity of a given sample is pores per linear inch(ppi). Numerous porous substrates are readily available, includingfiberglass mats, non-woven polyester webs, and various foams. In oneembodiment, the porous substrate provided in step 100 is an open-cellfoam, such as a reticulated polyurethane foam. Common applications offoam substrates used to filter air flow in electronic equipmentapplications can have 3 ppi to more than 20 ppi. Foams, such assynthetic plastic foams, also provide the desirable characteristics ofbeing compliant and resilient, offering the capability of “giving” andreturning to their original shape. In general, however, the poroussubstrate can be a commercially available, standard air filter.

In general, the EMI-absorbing solution provided in step 110 includes oneor more EMI-absorbing materials and a binding agent. In someembodiments, the EMI-absorbing solution also includes highly conductivematerial, such as copper or aluminum. EMI-absorbing materials areselected to suppress the transmission of electromagnetic energy, forexample, by converting the electromagnetic energy into another form ofenergy, such as thermal energy. EMI-absorbing materials may exhibitdielectric or magnetic properties, or a combination of both. Someexamples of EMI-absorbing materials include carbon, carbon fibers,alumina (Al₂O₃), sapphire, silica (SiO₂), titanium dioxide (TiO₂),ferrite, iron, iron silicide, graphite, and composites with differentcombinations of iron, nickel, and copper. The aforementionedEMI-absorbing materials are generally solids over anticipated ambientoperating temperatures and pressures. As such, the EMI-absorbingmaterials are generally prepared as particles suitable for suspensionwithin the binding agent.

Various U.S. patents describe lossy materials and their uses. See, forexample, U.S. Pat. No. 4,408,255 issued to Adkins, U.S. Pat. No.5,689,275 issued to Moore et al., U.S. Pat. No. 5,617,095 issued to Kimet al., and U.S. Pat. No. 5,428,506 issued to Brown et al., thedisclosures of which are herein incorporated by reference in theirentirety. Some manufacturers of lossy materials are R&F Products of SanMarcos, Calif.; ARC Technical Resources, Inc. of San Jose, Calif.; TokinAmerica, Inc. of Union City, Calif.; Intermark-USA, Inc. of Long IslandCity, N.Y.; TDK of Mount Prospect, Ill.; and Capcon of Inwood, N.Y.

The binding agent adheres the EMI-absorbing material to a substrate,such as the porous substrate. In some embodiments, a binding agent isselected that cures with a resilient consistency. In one embodiment, forexample, the binding agent is an elastomer, such as a resin binder. Inother embodiments, the binding agent is a rubber, such as a naturallatex rubber (for example, Stuart 1584), a synthetic rubber, such asstyrene-butadiene rubber (SBR), or a proprietary binder. Binders havinga resilient consistency adhere the EMI-absorbing material to the poroussubstrate, while allowing the porous substrate to remain flexible orsupple. In other embodiments, however, a binding agent is selected thatcures with a less resilient or even rigid consistency. One example of arigidly curing binding agent is an epoxy resin.

The application step (step 120) applies the EMI-absorbing solution tothe porous substrate. In one embodiment, the porous substrate is dippedin a bath of the EMI-absorbing solution. In another embodiment, theEMI-absorbing solution is applied to the porous substrate as a paint,for example, by either a brush, roller, or spray applicator. In yetother embodiments, the EMI-absorbing solution is applied to the poroussubstrate as an ink, for example, by one or more applicators bearing theEMI-absorbing solution. Generally, the EMI-absorbing solution is appliedliberally to the porous substrate, such that excess solution isthereafter removed.

The removal of excess EMI-absorbing solution (step 130) primarilyassures that the pores of the substrate treated with an application ofthe EMI-absorbing solution remain substantially open, thereby ensuringthat the substrate remains functional as an air filter. In oneembodiment, removal of the excess EMI-absorbing solution is accomplishedby squeezing or otherwise compressing the treated substrate. Forexample, the treated substrate can be drawn through a roller, such asone formed between two opposing cylindrical rollers, or a singlecylindrical roller opposing a rigid planar surface or plate. In otherembodiments, removal of the excess EMI-absorbing solution isaccomplished by forcing or drawing air through the treated poroussubstrate. The air can be forced through the treated substrate byapplying a positive pressure at a first surface of the substrate.Alternatively, air can be drawn through the treated substrate by drawinga vacuum on one side of the substrate. The removal of excessEMI-absorbing solution can be accomplished by a combination of theaforementioned methods.

The curing step (step 140) allows the applied finish of theEMI-absorbing material and binding agent to set. In some embodiments,the finished substrate can be air-cured at ambient room temperature. Inother embodiments, the finished substrate can be cured at elevatedtemperatures, for example in an oven.

In some embodiments, a fire retardant, such as a phosphate or antimonytrioxide, is optionally applied to the substrate (step 160) to meetstringent flammability standards. One such flammability standard is theUL94V0 vertical flame test, described in detail in UnderwritersLaboratories Standard 94, entitled “Tests for Flammability of PlasticMaterials for Parts in Devices and Appliances,” 5^(th) Edition, 1996,the disclosure of which is incorporated herein by reference in itsentirety. In some embodiments, a fire retardant is applied in the samemanner as described above for the EMI-absorbing solution (steps 100through 140). In other embodiments, additional treatments, such asfungicides, are similarly applied.

Referring now to FIG. 2A, a perspective view is illustrated depicting afree-standing, planar, EMI-absorbing air filter 200. In general, theplanar filter 200 defines an arbitrarily shaped cross section 210 (shownas a rectangle) having a predetermined thickness 205. There are noparticular constraints on the thickness 205, however common values rangefrom about 0.1 inch to 0.5 inch or more. The size of the cross section210 is generally determined by the application, typically being largerthan the air-vent opening to which it is affixed. FIG. 2B illustrates aperspective view of a framed configuration 208 including a planar,electromagnetic-interference-absorbing air filter 200 configured withina frame 210. The frame 210 provides rigidity and can include structurefor fastening the framed filter 208 to an equipment housing (not shown).For example, the frame 210 can include mounting holes 212 through whichfasteners are inserted to secure the framed filter 208 to the equipmenthousing.

As discussed above, the EMI-absorbing material is generally mosteffective at higher frequencies (for example, above 1 GHz). In someapplications, however, particularly where the cross section of the airfilter is relatively large (for example, greater than about 10 cm), theEMI-absorbing filter 200 can be combined advantageously with a lowfrequency EMI-mitigating means. Illustrated in FIG. 3 is a perspectiveview depicting a combination EMI/air filter. The combination filter 300includes an EMI-absorbing air filter 200, as described above, and anelectrically conducting layer 310. The conducting layer 310 is anelectrical conductor, such as aluminum or copper, with an array ofapertures through which air can flow. The conducting layer 310 can beformed from a rigid plate or from a screen. In some embodiments, theconducting layer includes a conductive coating applied to the filter200. The conductive coating generally consists of a highly conductivematerial, such as copper, aluminum, or gold. The conductive coating canbe prepared as a paint or ink and applied to the filter 200 by dipping,brushing or spraying. Alternatively, the conductive coating can beprepared as particles and applied to the filter 200 in a sputteringprocess.

The combination air filter 300 can be optionally mounted within a frame210 (illustrated in partial cutaway). The frame 210 offers rigidity andalso assists in forming a positive electrical ground from the conductinglayer 300 to the equipment housing. The frame 210 itself can beconducting, thereby providing electrical bonding between the conductinglayer 310 and an equipment housing. Alternatively, the frame can benon-conducting, forming an electrical bond by compressing the conductinglayer 310 against the chassis. Generally, the frame 210 includes afastening means 320, such as a mechanical fastener (for example, ascrew, a rivet, and the like).

Referring to FIG. 4, an alternative embodiment of a combination EMIfilter 400 is shown. A perspective view of the combination EMI filter400 is illustrated depicting an EMI absorbing air filter 200 combinedwith a waveguide-below-cutoff layer 405. The waveguide-below-cutofflayer 405 is formed from an electrical conductor, such as aluminum orcopper, and includes an array of apertures 410 (that is, waveguides)distributed across the filter's surface area. Each aperture 410 can beconstructed with arbitrary shapes, such as rectangular (shown),circular, and hexagonal. Each aperture 410 preferentially attenuateselectromagnetic radiation below a predetermined “cutoff” frequencycontrollable by the dimensions of the aperture 410. The apertures 410 ofthe waveguide-below-cutoff layer 405 allow air to flow to theEMI-absorbing air filter layer 200. As the EMI-absorbing layer 200attenuates higher frequencies, the resulting combination EMI filter 400attenuates a broader range of frequencies than either layer 200,405would otherwise attenuate alone.

In general, the EMI-absorbing air filters can be fashioned in anydesired configuration. FIGS. 5A and 5B illustrate exemplary non-planarapplications depicting embodiments in which the porous substrate uponwhich EMI-absorbing solution is applied is pleated 500, and tubular 510.

FIG. 6 illustrates one embodiment of a “dipping” manufacturing processfor forming the EMI-absorbing air filter. A container 600, such as atrough, holds an EMI-absorbing solution 610. A porous substrate 200 isthen immersed into the solution 610 thereby allowing the solution 610 tocompletely cover and penetrate the porous substrate 200. The substrate200 is then drawn from the solution 610 through a wringer 620. Thewringer 620, shown as a dual cylindrical roller assembly compresses thesubstrate 200 by a predetermined amount to remove excess solution 610and to ensure that the solution 610 is forced into the interior of thesubstrate 200.

FIG. 7 illustrates an alternative embodiment of a “spraying”manufacturing process for forming the EMI-absorbing air filter 200. Oneor more sprayers 700′, 700″ (generally 700), spray the EMI-absorbingsolution 710 onto the porous substrate 200. Generally, any type of sprayapplicator 700 known to those skilled in the art can be employed (forexample, pneumatic, mechanical, aerosol, etc.). The sprayer(s) 700 applya liberal coating of the EMI-absorbing solution 710 to completely coverand penetrate the porous substrate 200. The substrate 200 is next drawnthrough a wringing device, such as a dual cylindrical roller assembly730. The wringing device 730 compresses the substrate 200 by apredetermined amount to remove excess solution 720 and again to ensurethat the solution 710 is forced into the interior of the substrate 200.

FIG. 8 illustrates test measurement results relating to the EMIperformance of a sample EMI-absorbing air filter. The EMI-absorbing airfilter test sample was formed by applying a carbon-based absorber in anelastomer binder to an open-cell reticulated polyurethane foam planarsubstrate. The sample substrate was formed as a 0.25-inch thick sheethaving approximately 20 ppi. The sample was treated with a double carboncoating and flame retardant as described above. The electromagnetictransmission loss was measured across the filter over the frequencyrange from about 2.0 GHz to about 18.0 GHz. The resulting sampledemonstrated a measured attenuation of more than 20 decibels (dB) abovea frequency of about 4 GHz.

As the EMI-absorbing air filter must also function as an air filter, itis important that the filter allow sufficient air flow after beingtreated with the EMI-absorbing material and, optionally, with othercoatings, such as a flame-retardant coating. One measure of the airfilter's air flow performance is pressure drop versus air flow. Adiscussion of an exemplary test setup for measuring the air flowperformance, as well as measured air flow test results, are providedherein as an appendix and incorporated herein. Generally, any reductionin air flow resulting from the application of the one or more coatingsis controlled to reduce air flow by no more than a predetermined amount(for example, a difference in pressure drop for the same air flow of notmore than 10%).

Having shown exemplary and preferred embodiments, one skilled in the artwill realize that many variations are possible within the scope andspirit of the claimed invention. It is therefore the intention to limitthe invention only by the scope of the claims, including all variantsand equivalents.

APPENDIX Airflow Test Report

Objective:

This test compares the airflow characteristics of a non-shielding airfilter material to absorber-treated air filter materials.

Part Description:

A “baseline” air-filter material has been selected to represent anexemplary electronic-equipment dirt and dust filter. The baseline filterconsists of an open-cell polyurethane foam having approximately 20 ppiand a sample thickness of 0.25 inch.

A first sample reference “T-15” represents an EMI-absorbing air filterhaving a double coating of carbon and a flame-retardant treatment. TheT-15 sample has been formed using an open-cell polyurethane foam,approximately 15 ppi and a sample thickness of 025 inch.

A second sample reference “R-20” represents an EMI-absorbing air filterhaving a double coating of carbon and a flame-retardant treatment. TheR-20 sample has also been formed using an open-cell polyurethane foam,having approximately 20 ppi, and again having a sample thickness of 0.25inch.

Test Method:

Airflow testing was conducted in accordance with air-permeabilitystandard, ASTM D737, described in the American Society for Testing andMaterials Annual Book of ASTM Standards. The test set-up, arepresentation of which is shown in FIG. 9, consisted of a 6 inch'6 inchsheet metal duct 900 with metal flanges at each end (not shown). A firstend of the duct 902 was sealed against an opening in a plenum chamber910 using suitable fixtures and sealant. The EMI-absorbing air filtersample under test was attached to a second end of the duct 912 andsealed in a manner preventing leakage from the sides. A pressure tap 920was made on the duct at a distance of 18 inches from its second open end912. A plenum chamber outlet 930 was connected to the suction side of acentrifugal blower (vacuum pump) 940 via a series of valves 950 and anairflow-metering device. Calibrated instrumentation was used inmeasuring the test parameters.

Test Results:

FIG. 10 illustrates the resulting test data in graphical form comparingthe performance of the absorber-treated foams (T-15, R-20) to theuntreated baseline foam filter. The graph includes a vertical axisrepresenting “static pressure” (measured in inches of water) and ahorizontal axis representing “airflow” (measured incubic-feet-per-minute per square inch of vent panel, CFM/in²). Testresults for the untreated baseline foam and two samples of treated foamare illustrated on the graph. The test results demonstrate that thestatic pressure increases with increasing airflow for all three samples.This gradual increase in static pressure is due to the inherentresistance to airflow that the test panel offers to the air stream.

Conclusions:

The results indicate that there is virtually no difference between theuntreated baseline filter foam and the R-20 absorber filter foam. Asexpected, the T-15 absorber filter foam exhibits greater air flow thanthe baseline and R-20 samples. This is due to its cell structure beingmore open with 15 ppi as compared to 20 ppi for the other two testsamples. The data indicates that the airflow characteristics of the R-20sample should be similar to the baseline samples, while also providingEMI absorption.

1. An air filter having electromagnetic-energy absorptivecharacteristics, the filter comprising: a porous substrate; and anelectrically absorptive material applied to the porous substrate,wherein the electrically absorptive material is distributedsubstantially uniformly through the porous substrate.
 2. The air filterof claim 1, wherein the electrically absorptive material comprises anelectrical absorber and a binding agent.
 3. The air filter of claim 2,wherein the electrical absorber is selected from the group consisting ofcarbon, carbon particles, carbon fibers, alumina, sapphire, silica,titanium dioxide, ferrite, iron, iron silicide, graphite, and compositesof iron, nickel and copper.
 4. The air filter of claim 2, wherein thebinding agent is selected from the group consisting of an elastomer, arubber and an epoxy.
 5. The air filter of claim 2, wherein theelectrically absorptive layer further comprises a highly conductivematerial.
 6. The air filter of claim 5, wherein the highly conductivematerial is selected from the group consisting of copper and aluminum.7. The air filter of claim 1, further comprising a fire-retardant layer.8. The air filter of claim 7, wherein the fire-retardant layer comprisesa fire retardant selected from the group consisting of phosphates andantimony trioxide.
 9. The air filter of claim 7, wherein thefire-retardant-treated porous substrate passes a self-extinguishingvertical burn requirement in accordance with Underwriters LaboratoriesStandard
 94. 10. The air filter of claim 1, wherein the porous substratecomprises an open-cell reticulated polyurethane foam.
 11. The air filterof claim 10, wherein the foam comprises at least about 10 pores perlinear inch.
 12. The air filter of claim 1, wherein the porous substratecomprises a fiberglass mat.
 13. The air filter of claim 1, wherein theporous substrate comprises a non-woven polyester web.
 14. The air filterof claim 1, further comprising an electrically conductive layer.
 15. Theair filter of claim 14, wherein said electrically conductive layer is anelectrical conductor having an array of apertures through which air canflow.
 16. The air filter of claim 14, wherein said electricallyconductive layer is a conductive coating applied thereto.
 17. The airfilter of claim 14, wherein the electrically conductive layer comprisesa honeycomb.
 18. The air filter of claim 1, further comprising a framefixedly attached to the porous substrate, wherein the frame providesphysical support for the porous substrate.
 19. The air filter of claim1, wherein the porous substrate comprises a sheet having a thicknessless than about 0.5 inches.
 20. The air filter of claim 1, wherein theporous substrate provides at least 20 dB of attenuation toelectromagnetic energy substantially occurring at frequencies at leastbetween about 4 GHz and 18 GHz.
 21. A method for producing an air filterhaving electromagnetic-energy-absorptive characteristics comprising thesteps of: providing a porous substrate having a first side and a secondside; and applying an electrically absorptive solution to the poroussubstrate, wherein the electrically absorptive solution is distributedsubstantially uniformly through the porous substrate.
 22. The method ofclaim 21, wherein the applying step comprises the sub-steps of:providing an electrically absorptive solution comprising an electricalabsorber and a binding agent; immersing the porous substrate into theelectrically absorptive solution, causing the electrically absorptivesolution to penetrate the porous substrate; extracting the immersedporous substrate from the electrically absorptive solution; removingexcess electrically absorptive solution from the extracted poroussubstrate, thereby leaving a substantially uniform distribution ofelectrically absorptive solution through the porous substrate; andcuring the electrically absorptive solution.
 23. The method of claim 22,wherein the electrical absorber is selected from the group consisting ofcarbon, carbon particles, carbon fibers, alumina, sapphire, silica,titanium dioxide, ferrite, iron, iron silicide, graphite, and compositesof iron, nickel and copper.
 24. The method of claim 22, wherein thebinding agent is selected from the group consisting of an elastomer, arubber and an epoxy.
 25. The method of claim 22, further comprising thestep of forcing air through the porous material during at least one ofprior to curing and curing, thereby ensuring that pores remainsubstantially unblocked.
 26. The method of claim 25, wherein the step offorcing air through the porous material comprises drawing a vacuum. 27.The method of claim 21, wherein the step of removing excess electricallyabsorptive solution comprises squeezing the extracted porous substrate.28. The method of claim 21, wherein the step of applying an electricallyabsorptive solution is repeated.
 29. The method of claim 21, furthercomprising the step of applying a fire-retardant layer.
 30. The methodof claim 29, wherein the fire-retardant layer comprises a fire retardantselected from the group consisting of phosphates and antimony trioxide.31. The method of claim 21, wherein the applying step comprises:providing an electrically absorptive solution comprising an electricalabsorber and a binding agent; spraying the electrically absorptivesolution onto the first side of the porous substrate; removing excesselectrically absorptive solution from the sprayed, porous substrate,thereby leaving a substantially uniform distribution of electricallyabsorptive solution through the porous substrate; and curing theelectrically absorptive solution.
 32. The method of claim 31, furthercomprising the step of spraying the electrically absorptive solutiononto the second side of the porous substrate.
 33. The method of claim21, wherein the air-flow characteristics of the porous substrate aresubstantially equivalent before and after the application of theelectrically absorptive solution.
 34. The method of claim 21, wherein areduction in air-flow capacity of the porous substrate when comparedbefore and after the application of the electrically absorptive solutionis preferably less than 25%.
 35. The method of claim 21, wherein areduction in air-flow capacity of the porous substrate when comparedbefore and after the application of the electrically absorptive solutionis more preferably less than 15%.
 36. The method of claim 21, wherein areduction in air-flow capacity of the porous substrate when comparedbefore and after the application of the electrically absorptive solutionis even more preferably less than 10%.